For George H.W. Bush, it was liberating Kuwait
and not pushing on to Baghdad. Bill Clinton's might have been Northern Ireland peace
were it not for the Monica Lewinsky scandal. For George W. Bush, it was Afghanistan and Iraq. For Barack Obama, failing to follow through on a "red line" crossed
will haunt him for years.
These are moments of such magnitude that the world pauses to watch in horror, expectation, appreciation or disgust. They arrive, often unheralded, and are impossible to dodge.
Whether he knew it or not, Trump may as well have been framing the yardstick by which his allies will measure him when, standing beside King Abdullah II of Jordan
this week in the White House Rose Garden, he described the chemical attack in Syria. "These heinous actions by the Assad regime cannot be tolerated," Trump told the world.
Had he turned his back on the attack, refused to answer questions sufficiently from reporters on what he thought of the slaughter, it could have equally been a legacy-defining moment, setting America on a course of unparalleled isolationism in its modern history.
But he didn't. He said he owns the Bashar al-Assad problem: "I am responsible now." And like Obama before him, he may have dared Assad to do it again.
Trump's UN ambassador, Nikki Haley, warned that if the United Nations doesn't act, the United States might "act alone."
Barely 70 days in office, Trump has arrived at the moment some presidents wait years to come their way.
While he gave the appearance of speaking from the heart about the gassing of "beautiful little babies," it was less clear if his comments represented a change in policy, or were just retorts to tough questions.
But the world didn't have to wait long to find out. Fifty-nine American Tomahawk cruise missiles took off from US warships overnight and tore into the air base
from where the US believes Assad's air force took off to drop its chemical weapons.
For America's allies and other international observers, there may have been a sense this week that Trump had not only caught up with the Obama position on Assad but leapfrogged it. There may also be a sense that Trump has finally smelt the coffee on Russia, having previously sounded softer than recent US presidents when it came to Vladimir Putin.
After months of concern he wouldn't be tough on Assad's biggest backer, there will be relief that after the chemical weapons attack, Trump's ambassador to the United Nations and his secretary of state publicly challenged Russia to call Assad to heel.
But there is still every reason to believe that Russia won't listen -- even after the US cruise missiles.
Trump is left at odds with Putin. The gloves are already coming off, with Russia accusing the United States
of "an attempt to distract from the mounting civilian casualties in Iraq" and accusing Trump of violating Syria's sovereign territory."
Syria is a complex place, turned inside out by conflict where a myriad of local and international interests compete for ascendancy, churning out casualties on an unimaginable scale.
This is a conflict in which Iran (a Republican nemesis) and Russia (a former Trump favorite) align with Assad, one of recent history's most murderous leaders, to stifle democratic street protest-turned-civil war.
This is a conflict that pitches fighter jets and attack helicopters against civilians and terrorists alike.
Trump's solution to the Syria crisis was once simple: Take out the terrorists. He promised in his election campaign to go big on ISIS.
One of his first executive orders was to his defense secretary, James Mattis, compelling the former general to come up with a new strategy to tackle ISIS
inside 30 days.
Mattis delivered it a day early, but since then, no details have been made public.
In London last week, standing shoulder to shoulder with his British counterpart, Michael Fallon, Mattis was asked by a journalist: "Is your policy in Syria that Assad must go?"
Mattis, who is fast becoming Trump's most able diplomat, was deft in his answer: "I would say on the Assad issue, we are working on this one day at a time as we throw Daesh (ISIS) on to the back foot."
In less than a week, Trump has traveled diplomatic light-years. In the Rose Garden he said he was proud of his flexibility. But his allies may still be contorting themselves, trying to work out if even after the US strike his latest position will stick.
Early reactions to the strike from France and Germany are positive and almost convey a sense of relief.
But it's the next card Trump plays -- how he handles his new determination that Assad can no longer act with impunity -- that will shape his legacy.
A path in history has been set by the chemical attack this week, a jolt significant enough to knock Trump clean off one track and on to another. He has stated his course and for now seems to be following it. But then he has so many times before.
It's this unpredictability that will be worrying Putin as much as it does Trump's allies. The Russian leader is being forced in to a recalibration, no longer facing off with a predictable leader, as Obama had become.
The contours of Trump's legacy-defining moment are just beginning to take shape.
In his book "The Art of the Deal," Trump makes a virtue of his bluster.
Now has come the time his mettle will begin to be measured, in perpetuity.